In high-precision optical systems, alignment isn’t optional; it’s essential. Even minor shifts can impact performance, especially in spectroscopy, where lasers must be precisely focused to detect chemical signatures. Without this key requirement, the signals being measured will have a poor signal-to-noise ratio. This could potentially lead to weak or missed peaks, resulting in false or incomplete conclusions from the data analysis. Recognising this challenge, Dr Alex Poppe, a senior software engineer at IS-Instruments (ISI), developed an intelligent algorithm that automates the laser alignment process. Auto-alignment for lasers offers time-saving and enables performance in environments where human access is limited or hazardous.
Smarter, Faster Alignment
The algorithm was conceptualised in a couple of days, and it took a couple of weeks to implement and test. After conversion to an executable format for integration with LabVIEW, it was deployed to run with ISI’s gas Raman spectrometer. Its job is to automatically realign the laser to the point of highest optical power, without human input.
The spectrometer can be aligned by using piezo actuators to move a lens, which in turn steers a laser spot with respect to a hollow-core fibre to a position where the laser power can be maximised. This is identified using a power meter. A standard way to align a spectrometer is to perform a raster scan across the entire field of view of the fibre cross-section, which maps out the whole laser power “landscape” to determine the optimal position for alignment.
The raster scan approach, whilst effective, is prohibitively slow. Hence, from the perspective of sustainability, another alignment algorithm was necessary. This new algorithm was brainstormed [see the image below], and from an original concept, Dr Poppe and Charles Warren, fellow software developer, produced a novel geometry that is both fast and efficient.
“The algorithm was designed to be robust enough to operate even when background noise found at lower power levels makes signal detection challenging,” says Dr Poppe. “We included a fallback strategy; if the system can’t find a clear maximum, it ‘shunts’ the laser to a new region and begins again. That allows us to operate in more complex or degraded environments.”
The algorithm can be succinctly summarised as performing a “zigzag” pattern from an initial starting point, alternating between horizontal (X) and vertical (Y) shifts, as it attempts to move “uphill” in the laser power landscape towards a target power level. A redundancy was implemented to handle cases of extreme misalignment, where the algorithm would have otherwise failed to align within the allotted time limit. This was achieved by “shunting” the laser to a new location, allowing it to escape from a local maximum and begin the zigzagging pattern again.
Each time the laser is activated, the alignment algorithm is repeated, quickly realigning the spectrometer to optimal levels, as the natural drift in laser power is minimal over short intervals.
It is challenging to quantify the amount of time saved by this algorithm compared with the raster scan. Either algorithm could theoretically begin at an optimal power, rendering such a comparison meaningless. However, this new algorithm will, on average, align significantly faster than a standard raster scan, as most of its movements are towards a higher laser power. Conversely, a raster scan must meticulously move to every position until it reaches an optimal point. Thus, many of the movements it makes may be inefficiently moving to a lower laser power than it was previously.
Built for Hostile Environments
ISI is experienced in deploying Raman spectroscopy in hazardous environments. One of the significant advantages of the auto-alignment algorithm is that it can function without manual adjustment, making it ideal for remote deployment in hazardous environments.
This algorithm has already proven useful in the GRADE project, funded by the UK Atomic Energy Authority as part of its Fusion Industry Programme (FIP). This programme is encouraging the development of fusion energy instrumentation and technologies to drive the growth of the industry. ISI, with the Optoelectronics Research Centre and Amentum, is developing a gas Raman spectrometer to identify isotopologues of hydrogen, including tritium, a radioactive variant crucial to fusion research.
The radioactive environment where the gas Raman instrument operates provided an ideal testing ground for the laser alignment algorithm. In the nuclear sector, there is a strong effort to eliminate human operators, and this new algorithm supports this goal. – Jessica Gabb, Scientist & ISI Project Manager for GRADE.
Looking Ahead
Compared to older scanning methods such as the Rasta scan, which is slower and more intrusive, Dr Poppe’s approach is faster and more adaptive. ISI is now considering exploring the possibility of combining this method with spiral scan models to further accelerate the alignment process.
